Reconciling the Differences Between Tolerance Specification and Measurement Methods

The ASME Y14.5M standard has defined different types of tolerances that can be applied to a feature to achieve the required functionality. Each tolerance defines a zone within which the feature under inspection must lie. The conformance of the parts to these tolerances is checked by manual measurements or a CMM. But it has been observed that the measurements between different CMMs do not match. There are two generally accepted reasons for this discrepancy. The first one is the measurement uncertainty in CMM software. This problem was addressed by NIST by developing reference softwares for feature fitting algorithms. And the second one is the distinct choice of algorithms for fitting substitute feature to the data points measured from CMM. Feature fitting algorithms used in CMMs are often based on their mathematical convenience rather than the interpretation of definitions in the GD&T standard. Our research is focused on identifying that normative algorithm that is best to be used for each type of tolerance. Each normative algorithm is identified as the one to best represent the interpretation of geometric control as defined by the Standard and on the manual methods used for the measurement of a specific tolerance type.